I had this crop up recently. on a 94 Z28 style. There are placement for all 3 cooling relays in the fuse panel. no rats have chewed things up, no bad connections. clearing the fault code with relays in or out does nothing, it comes back within a few seconds with the engine running. PCM always starts up with the fans coming on immediately when the ignition is on. I'm not sure how the PCM knows there is an error code 77 since I see no feedback circuit or method to the microcontroller.
Using the new 9495 PC tool, other actuators (gauge lights, idle speed) all work, but turning the fans off and on does nothing, they are always engaged (short to ground through A10/A11.

What am I missing?

http://shbox.com/1/fan_schematic_1995.jpg

Code 77- Cooling fan control relay circuit. Check relay circuit for open or shorts to ground. If no wiring problems are found, replace relay. Be careful as intermittent short or open can only be detected by wiggling the wires and observing any voltage fluctuations to the affected circuit.

 

The PCM monitors the voltage on the driver that supplies the ground to turn the fans on. If the voltages does not match what is expected for the commanded state, the code sets. If the fan is supposed to be "off", the voltage on the circuit will be near battery voltage. If the fan is supposed to be "on", the voltage on the circuit will be near 0V.

The code sets when the incorrect voltage is sensed on the circuit for 26 seconds.

Which 94 fan arrangement do you have - "early" 2 relay, one for fan #1 and one for fan #2? Or "late" a set of three relays that turns both fans on low, then to high? The 94 factory manual only addresses the "early" arrangement with respect to DTC 77. I suspect the 95 FSM addresses the three relay setup.

Appears the code is setting because the fans are running when they are not supposed to - e.g. when you turn the key to on.

The fans can only run if there is a ground being supplied to the relay. It sounds like you have a fault to ground somewhere on the wires from the PCM to the relays.

 

Thanks for the complete reply. I'll recheck for fault-to-ground along A10/A11 circuits.

I assume I have the 3 fan setup since the relay box contains labels, locations, and relays for all three relays even though it is a '94 car.

When ignition is on, all three relays energize immediately.

 

The diagnostic logic tree in the manual does have a branch that ends with "faulty PCM".

 

I would use the ohmmeter/continuity function of a multimeter to monitor the resistance to ground of those two lines (at D2 and J2 of the relay sockets with the relays removed) while unplugging things, like C100 and PCM connector A.

 

As usual a simple answer comes to a seemingly complex problem. The fan override that was done by the police department nearly 20 years ago was to connect the blue and green wires at the relay box, splice in a black wire and route the black wire all the way into the passenger compartment through a SPST switch.
Somewhere along this was indeed shorted to ground with the switch open. Switch was fine, wire was routed under the PCM and it had worked its way through the insulation of the wire and shorted it to ground under the PCM.

Error 77 is what happens when the fan override switch is on, regular fan on/off by the PCM still works even with the error active I'm guessing since its been fine for years. But while I was in there I disabled the "Check 77" from the PCM constants table section, didn't see any issues running the vehicle and the code didn't come up anymore, but I didn't run it hot enough yet to see if the regular fan operation occured from the PCM.

Thanks for the assistance, got it sorted. Error 77 really does mean that the PCM output pins are grounded when it does not expect it.

 

Well there you go. Great follow-up and troubleshooting. Not sure about the need to delete the code, but congratulations.